Joint compounds or tape joint compounds are paste-like materials typically used in the construction of interior walls. In general, they are applied at the joint between two adjacent wall board panels or in cracks or holes in existing walls. The purpose of the joint compound is then to form a monolithic and regular surface for subsequent painting.
In general, there are two types of joint compound: 1) drying and 2) setting.
Drying joint compounds harden when they dry by evaporation; setting joint compounds are mixed with water immediately before use and sets through a chemical hardening process rather than evaporation.
Drying joint compounds are typically ready-to-use (or ready-mixed), that is in the form of aqueous pastes containing inert inorganic fillers; limestone (a sedimentary rock composed largely of calcium carbonate minerals) is the predominant inorganic filler in drying joint compounds.
Water is already well mixed with the inorganic filler, which is inert, i.e. not able to react with water, and upon application, the water evaporates to the atmosphere.
Drying joint compounds can be also provided in dry form, but usually they are sold as ready-to-use mix.
On the other hand, setting joint compounds are only sold in powder form and comprise inorganic fillers which react with water (hydraulic binders); water must be always added at the job site.
Typically, all joint compounds contain a filler, a binder and a thickener.
The conventional fillers are calcium carbonate, calcium sulfate dihydrate (gypsum), and calcium sulfate hemihydrate (plaster of Paris).
Calcium carbonate and calcium sulfate dihydrate (gypsum) are inert inorganic filler and can be used in ready-to-use drying joint compounds.
Since it reacts with water, calcium sulfate hemihydrate is used only in setting type joint compound. Usually, in current construction practices, it is preferred to use a ready-mixed or pre-wetted joint compound, which contains either a calcium carbonate and/or gypsum filler.
As mentioned above, most commercial joint compound formulations contain a thickener. Thickeners are used in numerous products particularly for increasing viscosity and imparting the required rheological properties to the products.
The thickeners for joint compounds can be natural polymers, such as guar or xanthan; synthetic polymers, such as polyacrylate or polyurethane based thickener; or semi-synthetic polymers, or chemically modified natural polymers, such as cellulose derivatives; all exhibiting the specific characteristic of bonding and coordinating a large amount of water once they are dissolved in water.
Among the chemically modified natural polymers, the thickeners of choice have been for a long time the derivatives of cellulose, including carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), ethyl hydroxyethyl cellulose (EHEC), methyl cellulose (MC), hydroxylethyl methyl cellulose (MHEC), hydroxypropyl methyl cellulose (HPMC) ethers alone and blends of them.
Hydroxypropyl guar is also known as thickener for joint compounds, as described in U.S. Pat. No. 5,512,616. Like the cellulose derivatives it modifies the viscosity and rheology of the joint compounds by imparting pseudoplastic behaviour to the paste.
In recent years, associative chemically modified natural polymers have been used, such as hydrophobically modified hydroxyethyl cellulose. This technical solution has been described in U.S. Pat. No. 5,258,069 or U.S. Pat. No. 5,102,462. Now, it has been surprisingly found that C3-C24 hydrophobically modified polygalactomannans impart improved workability and long-term stability to joint compounds, in particular to ready-to-mix drying joint compounds, compared to the prior art thickener, while simultaneously maintaining water retention, sag resistance, and other important properties.
As far as the Applicant knows, no one has described the use of C3-C24 hydrophobically modified polygalactomannans in joint compounds, in particular in ready-to-mix drying joint compounds.
In the present text, with the expression “C3-C24 hydrophobically modified polygalactomannan” we mean a polygalactomannan which has been modified with the introduction, as substituent, of an unsubstituted linear or branched alkyl chain with from 3 to 24 carbon atoms.
With the expression “molar substitution” (MS), we mean the average number of hydroxyalkyl substituents on each anhydroglycosidic unit of the polygalactomannan, which can be measured, for example, by 1H-NMR.
With the expression “degree of substitution” (DS), we mean the average number of substituted hydroxyl groups on each anhydroglycosidic unit of the polygalactomannan, which can be measured, for example, by 1H-NMR.